This invention relates generally to nuclear chlorine substitution reactions of aromatic hydrocarbons and in particular to the formation of dichlorobenzenes from benzene or monochlorobenzene feedstocks.
Orthodichlorobenzene is useful as a solvent, heat exchange media and chemical intermediate for synthesis of herbicides. Commercial liquid phase nuclear chlorination processes producing dichlorobenzenes yield a product distribution wherein para-isomer considerably predominates over the ortho-isomer. It is desirable to adjust the ratio of dichlorobenzene isomers to promote formation of the ortho-isomer in the product.
The nuclear chlorine substitution of benzenes to form dichlorobenzenes is conventionally performed in a liquid phase process by contacting gaseous chlorine with benzene or monochlorobenzene in the presence of a ferric chloride or aluminum chloride catalyst, for example, as described by Wiegandt, H. F. and Lantos, P. R., et al., Industrial & Engineering Chem., Vol. 43, p. 2167-72, 1951. The ferric chloride liquid phase catalyzed dichlorobenzene formation typically has a ratio of para- to ortho- isomers of about 1.4 and above.
Substitution type chlorination reactions of benzene also produce some monochlorobenzene, metadichlorbenzene and poly-chlorinated (three or more chlorine atoms) benzenes. Monochlorobenzene may be recycled to the reaction zone. It is desirable to minimize the formation of the metaisomer because it interferes with the separation by distillation of the para-isomer. The polychlorobenzenes have marginal industrial value.
Manganese has been employed in chlorination processes. For example, manganese oxide has a described utility for producing a high yield of paradichlorobenzene (U.S. Pat. No. 2,046,411 issued to W. D. Ramage). In addition, manganese chloride is described as a chlorination promoter in the presence of ferric chloride (U.S. Pat. No. 3,214,482 issued to F. E. Caropreso).